The present invention relates to granules for formation of an electrode of an electric double layer capacitor, their manufacturing method, an electrode sheet, a polarized electrode, and an electric double layer capacitor using a polarized electrode.
Further, the present invention relates to a manufacturing method of a sheet-like polarized electrode for an electric double layer capacitor.
Electric double layer condensers (also called electric double layer capacitors) have farad-class large capacitance, are superior in the operating cycle characteristic, and can be charged quickly. For these reasons, electric double layer capacitors are used as backup power sources of electronic equipment and vehicular batteries (energy buffers) and for other purposes.
An electric double layer capacitor will be outlined below with reference to FIG. 4.
FIG. 4 is a sectional view showing the basic configuration of an electric double layer capacitor.
The electric double layer capacitor 101 shown in FIG. 4 is composed of a container 102 and a pair of carbon electrodes (polarized electrodes) 104 and a pair of collectors (collector members) 105 that are housed in the container 102. A separator 103 is interposed between the carbon electrodes 104. An ion-conductive electrolytic solution is provided in the container 102. The electric double layer capacitor 101 is a capacitor in which charges (indicated by “+” and “−” in the figure) that are generated at each of the boundaries between the carbon electrodes 104 (solids) and the electrolytic solution (liquid) and are separated from each other by a molecule-level distance are used as corresponding to a dielectric of an ordinary capacitor.
Electrolytic solutions used in electric double layer capacitors are generally classified into what is called water-soluble-type electrolytic solutions obtained by adding an electrolyte to dilute sulfuric acid and what is called organic electrolytic solutions obtained by adding an electrolyte to an organic solvent, and selection is made between these two types in accordance with a purpose. That is, electric double layer capacitors using a water-soluble-type electrolytic solution are low in internal resistance and hence are advantageous in terms of power density. On the other hand, organic electrolytic solutions can increase the breakdown voltage per unit cell and hence are advantageous in terms of energy density. Further, organic electrolytic solutions enable use of inexpensive and light metals such as aluminum.
In practice, as shown in FIG. 2, such electric double layer capacitors employ a laminated polarized electrode 9 (or 10) in which an electrode layer (electrode sheet) e and collector foil (metal foil) 11 (or 14) are bonded to each other via, if desired, a bonding layer.
As shown in FIGS. 1A and 1B, an electric double layer capacitor is formed by laminating the polarized electrodes 9 and 10 and a separator(s) s alternately and enclosing those in a container 2.
The electric double layer capacitor 1 of FIG. 1A is mainly composed of the cylindrical container 2, an electrode roll 3 that is a laminated body of the polarized electrodes 9 and 10 and the separators s and is housed in the container 2, and an electrolytic solution that is provided in the container 2 as a result of injection.
On the other hand, the electric double layer capacitor 1 of FIG. 1B is mainly composed of coin-type containers 2 and 2′ and a laminated body that consists of the polarized electrodes 9 and 10 and the separator s and is housed in the containers 2 and 2′. The separator s is impregnated with an electrolytic solution.
Polarized electrodes to be used in electric double layer capacitors are required to have a porous structure. Further, since they are used for a long time in various forms, they are required not to be prone to cracks or damage and to be high in shape retention.
There may occur an event that granules or particles of the electrode material such as carbon of the electrodes desorb to cause a voltage decrease or increase in internal resistance and the desorbed granules or particles of the electrode material are suspended in the electrolytic solution or interposed between the separators to disable long-term maintenance of a prescribed voltage.
A method for preventing coming-off of electrode formation granules in an electric double layer capacitor is disclosed in patent document 1, for example. That is, patent document 1 discloses a manufacturing method for manufacturing a polarized electrode of an electric double layer capacitor by forming a mixture of electrode materials that are a carbon fine powder (activated carbon), a conductive filler, a binder, etc. and a liquid lubricant into a sheet, removing the lubricant, and rolling the formed sheet along one or a plurality of axes.
Patent document 2 discloses an electrode sheet for an electric double layer capacitor in which the tensile strength of a polarized electrode is set to 0.13 Pa to prevent carbon granules as part of an electrode from coming off into an electrolytic solution.
[Patent Document 1]
JP-B-7-105316
[Patent Document 2]
JP-A-2001-267187
Because electric double layer condensers (capacitors) have large capacitance and are superior in the operating cycle characteristic, studies are now being made to use those as various backup power sources of automobiles etc. When used as backup power sources of automobiles etc., electric double layer capacitors should have a large capacitance. Therefore, polarized electrodes used in such electric double layer capacitors should assume long, sheet-like shapes. And various manufacturing methods are proposed for such sheet-like polarized electrodes.
The following method is known though it does not directly relate to problems to be solved by the invention. Activated carbon as a carbonaceous powder, carbon black as a conductivity-enhancing agent, and a fluororesin (PTFE (polytetrafluoroethylene)) as a binder, and ethanol are mixed together with a mixer. A resulting mixture is dried and then kneaded with a kneader. A resulting kneaded material is crushed with a mixer into granules of 2 mm or less in diameter. The granules are pressure-formed with parallel rolls into a sheet-like formed body (refer to patent document 3, for example).
[Patent Document 3]
JP-A-2001-35756
However, in the case of polarized electrodes produced by the methods of patent documents 1 and 2, whereas the desorption of the granules themselves constituting the electrodes in a completed capacitor is prevented to some extent, in a process of manufacturing a polarized electrode or in manufacturing an electric double layer capacitor by enclosing polarized electrodes in a container fine particles of an active material, a conductive filler, etc. are peeled off or desorbed and then suspended in an electrolytic solution or interposed between separators, which leads to reduction in the voltage maintenance ratio.
Incidentally, in manufacture of the above type of sheet-like polarized electrode, it is desired that the sheet not be prone to porosities or cracks and be high in tensile strength.